1. Field of the Invention
This invention relates to a process for preparing biphenyl-4,4'-dicarboxylic acid by oxidizing 4,4'-diisopropylbiphenyl or an oxidation intermediate thereof with molecular oxygen. Biphenyl-4,4'-dicarboxylic acid is useful as a raw material for the production of high-performance polyesters and aramid resins having high heat resistance and strength.
2. Related Art
The following processes have heretofore been known as preparation processes of biphenyl-4,4'-dicarboxylic acid.
(1) 4,4'-diacetylbiphenyl, which has been obtained by acetylating biphenyl, is oxidized with a hypochlorite [Ukr, Khim, Zh., 30, 938-941 (1964)].
(2) p-Bromotoluene is converted with magnesium into 4,4'-dimethylbiphenyl in ether, followed by oxidation with a cobalt catalyst and bromine catalyst in acetic acid [Zr, Prinkl, Khim, 40, 935 (1967)].
(3) Using iron chloride or aluminum chloride as a catalyst, biphenyl is reacted with a halogenated cyclohexane in nitrobenzene or carbon disulfide as a solvent to form 4,4'-dicyclohexylbiphenyl. It is then oxidized in the presence of a cobalt catalyst, manganese catalyst and bromine catalyst in acetic acid (Japanese Patent Laid-Open No. 16831/1982).
(4) Using BF.sub.3 as a catalyst, a 4-alkylbiphenyl is reacted with carbon monoxide in hydrogen fluoride as a solvent to form a 4-alkyl-4'-formylbiphenyl. It is then oxidized in the presence of a cobalt catalyst, manganese catalyst and bromine catalyst in acetic acid (Japanese Patent Laid-Open No. 174745/1985).
These conventional processes are however accompanied by the following problems.
The process (1) requires, as a catalyst for the acetylation, a great deal of aluminum chloride which is difficult to regenerate and is highly corrosive. In addition, the oxidation with the hypochlorite induces significant corrosion. It is hence difficult to practise this process industrially.
The process (2) obtains the starting material, 4,4'-diacetylbiphenyl, by a Grignard reaction which consumes expensive magnesium. This process is thus impractical.
The process (3) obtains the starting material, 4,4'-dicyclohexylbiphenyl by using iron chloride, aluminum chloride or the like, which is difficult to regenerate and is highly corrosive, as a catalyst like the process (1) and by employing as a solvent nitrobenzene or carbon disulfide which is very dangerous due to its high inflammability and toxicity. Its industrial practice is therefore very troublesome. Moreover, oxidation of the starting material results in an oxidative loss of the expensive cyclohexyl groups. It is hence difficult to practise this process industrially.
The process (4) makes use of hydrogen fluoride and BF.sub.3, which are very corrosive, in order to obtain the starting material, i.e., the 4-alkyl-4'-formylbiphenyl. It is therefore necessary to use an expensive and corrosion-resistant reactor. This process is accompanied by another drawback that the purification of the resulting product is cumbersome, since di-substituted derivatives other than 4,4'-derivative are also formed.
The above-described processes, which have heretofore been proposed as processes for the preparation of biphenyl-4,4'-dicarboxylic acid, are accompanied by their own drawbacks. A great deal of effrts has been exercised to overcome such drawbacks. No satisfactory effects have however been achieved to date. Accordingly, the high price of biphenyl-4,4'-dicarboxylic acid still remains as an obstacle for the expansion of its application field.